Polyfunctionalized organometallics are important intermediates in modern organic synthesis.[1]
One of the best preparative methods of these reagents is the halogen-metal exchange reaction. Whereas the Br/Li-exchange is a fast reaction which occurs at low temperature, the corresponding Br/Mg-exchange is considerably slower which is a severe synthetic limitation for several reasons:    (i) the exchange requires higher reaction temperature and therefore is not compatible with many functional groups,    (ii) the slow Br/Mg-exchange especially on electron-rich aromatic bromides is in competition with the elimination of HBr from the alkyl bromide also produced during the reaction (usually isopropyl bromide) and therefore, results in low yields. A catalysis of the Br/Mg-exchange would be a highly desirable process. Recently, the inventors have shown that highly functionalized aryl- and heteroaryl-magnesium halides can be readily prepared by using an iodine-magnesium exchange reaction.[2] As exchange reagent i-PrMgX (X═Cl, Br) proves to be most convenient. In some cases, this exchange reaction could be extended to some aryl and heteroaryl bromides when a powerful electron-withdrawing and (or) a chelating group was present to coordinate i-PrMgX and make the Br—Mg exchange “intramolecular”.[3]
Basically, the I/Mg-exchange reaction is an excellent method for preparing functionalized aryl and heteroaryl compounds. It has as main drawback the need of using sometimes unstable, often expensive or commercially not available organic iodides. The alternative of using aryl bromides as substrates for the Br/Mg-exchange is known, but was strongly limited to only highly reactive aryl bromides (bearing several electron-withdrawing groups) due to the low rate of the exchange reaction using either i-PrMgCl or i-Pr2Mg.
Therefore, it is a problem underlying the present invention to provide an improved method of preparing organomagnesium compounds. It is a further problem underlying the present invention to provide an organomagnesium compound, which has a higher reactivity with an electrophile (E+).
These problems are solved by the subject-matter of the independent claims. Preferred embodiments are set forth in the dependent claims.